For an image to be reproduced on a raster image device, the image must be sampled to provide raster scan data. Therefore, an image is scanned in a linear manner and sample points which are light may be represented by a 0, and sample points which are dark may be represented by a 1. This scan is repeated over the entire surface of the image to be displayed producing raster scan data in which each bit represents a sample point within the image.
The raster scan data is transmitted to a raster display device such as the printer shown in U.S. Pat. No. 4,031,519 by Findley. In such devices a laser beam writes out the image of the text by setting up a pattern of electrostatic charge on a belt or drum according to the raster scan data, thus the data is converted into an image.
When an edge of an image reproduced on such a device is at an oblique angle with respect to the direction of the raster scan, jagged spots are produced in the image. This problem is called aliasing. An anti-aliasing technique is disclosed in U.S. Pat. No. 4,079,367 by Yonezawa et al which shows detection of two adjacent pels positioned in an oblique direction and the insertion of an additional pel to smooth a jagged portion. Yonezawa et al use a sliding window to examine matrices of pels for such oblique pairs of pels. However, Yonezawa et al has limitations in anti-aliasing on more complex configurations of pels.
It is also known to write a gray or halftone halo along the periphery of images reproduced from raster scan data. This non-selective printing of a gray pel at all edges creates a halo around the image thereby smoothing the image and enhancing resolution. Further, Architectures and Algorithms for Parallel Updates of Raster Scan Displays; Satish Gupta; Carnegie-Mellon University, Department of Computer Science; December, 1981, teaches the selection of pel intensity depending upon the distance between the center of the pel and the edge of the image. Gupta also shows a table look up to make anti-aliasing computations more efficient. "Digital Typography", Charles Bigelow, Scientific American, August, 1983, also shows the selection of one of a plurality of gray levels to enhance edges.
It is also known to shift pels a half pel position when printing diagonals as described, for example, in "Character Edge Smoothing for Matrix Printing" by D. L. Ort in the Xerox Disclosure Journal, Volume 6, Number 1, January/February 1981.
A more precise technique which shifts selected pels one half position is shown in U.S. Pat. No. No. 4,232,342 by R. Sommer in which a pel is shifted or not shifted depending on its position relative to the desired contour.